1. Field of the Invention
The present invention is directed to a process for washing pulp which has been cooked in a batch digester using a modified kraft pulping process. The invention is particularly concerned with the use of multi-stage pulp washing within the digester itself.
2. Description of the Prior Art
There are numerous types of processes for batch digestion of wood chips in the manufacture of paper, usually taking place in a digester specifically built for that purpose. The digester is filled with the wood chips which are usually compacted therein. Hot solutions of sodium hydroxide alone or in admixture with sodium sulfide are then charged into the digester. The temperature of the digester is conventionally controlled through the introduction of steam. After the chips are maintained in contact with the cooking liquor for a predetermined period of time, a blow valve in the digester is opened to dump the contents into a blow tank.
In a previously disclosed modification of this basic pulping process, the hot black liquor is displaced from the digester by pumping washer filtrate into the bottom of the digester at the end of the cook. The hot black liquor leaves the digester through an extraction screen located in the top dome of the digester. Most of the displaced hot black liquor goes to the pressurized hot black liquor accumulator and the final volume, as the temperature drops, goes to a warm black liquor accumulator. Tests have shown that pulp washing equivalent to a one-stage filter is achieved in the hot black liquor displacement. Additional pulp washing after the pulp is blown from the digester is required to achieve an acceptably low level of black liquor remaining in the pulp. This modified process is disclosed in Bertil Fagerlund's U.S. Pat. No. 4,578,149 entitled "Process For Digesting Cellulosic Material With Heat Recovery".